my filter filled with the tiny little snails. the biggest ones measured around 1/4" and have black/dark color shell. I was worry at first that it might impact the water condition but as months passed by, they appeared to be harmless. I do believe they eat some the waste trapped by the japanese mats.

These are the same snails that give power plants trouble with their cooling system. As some of us in the beginning have added plats to our ponds not expecting those tiny little snails will become a head along the way...You can use Dimlin, PP and Quick Cure to rid yourself of these little headaches.

They do sneak in

I have a few apple snails for munching algae (in slow motion) but a few of the ittybittybuggers snuck in on some plants and the breeding frenzy began. Those things multuply so fast they make jackrabbits blush I did find that algaecide knocks them down pretty well (but I hate using the stuff). I think the sparrows, robins, and orioles may have been snacking on them too . (They like to bathe in the stream)